These studies will examine some of the enzymatic properties of endonucleas III and IV of E. coli and endonuclease V of bacteriophage T4. Purification of these enzymes to homogeneity will allow us to define exact substrates and products for the reactions catalyzed by endonuclease III on DNA containing modified pyrimidine residues, by endonuclease IV on apurinic and apyrimidinic DNA, and by endonucleases III, IV and V on DNA containing deletion loops. In vitro site directed mutagenesis will be used to obtain mutants for endonuclease III. Purification and characterization of the mutant endonuclease III enzymes will help define the number of active sites within the protein and DNA sequencing of wild-type and mutant genes may allow us to define the active sites. Base excision repair may be one of the major pathways by which cells repair damaged bases in DNA. Enzymatic activities for such repair have been found in a large number of prokaryotes and eukaryotes. A careful study of some of these enzymes will be useful in defining the exact fashion in which cells repair DNA damaged by ultraviolet and X-irradiation and chemicals. Such treatments are used to treat cancer, and are also known to promote cancer.